A nucleus with a nonspherical distribution of charge will possess an electric quadrupole moment. The interaction of this nuclear quadrupole moment with an electric field gradient imposed upon it by surrounding electrons can lead to broadenings on the order of kilohertz or even many megahertz in Solid State NMR (SSNMR) spectra of such nuclei. The linewidths achieved in the high resolution SSNMR of S=1/2 (non-quadrupolar) nuclei are typically 10-70Hz. We are studying a recently proposed experiment [Frydman, L., Harwood, J.S., J. Am. Chem. Soc. 117 (1995) 5367] that is capable of obtaining isotropic spectra of half-integer quadrupolar nuclei while also determining individual quadrupolar couplings of the resolvable sites. This technique exploits multiple quantum (MQ) coherences during magic angle spinning (MAS), and is known as the MQMAS experiment. With this technique we have presented the first high resolution spectrum of 17O reported in the literature [Wu, G., Rovnyak, D., Sun, Q.Q., Griffin, R.G., Chem. Phys. Lett., 1996, in press], by studying the phosphate oxygens in hydroxyapatite, a model for bone in vertebrates. With the compound sodium dihydrogen phosphate, which has three distintct 23Na sites, we demonstrated the utility of a two dimensional presentation of MQMAS for resolving chemically distinct sites along one frequency axis and the individual quadrupolar MAS lineshapes along the other. While it is well known that NMR peaks may be integrated to reveal the relative populations of nuclei giving rise to each signal, this was not the case for MQMAS when it was first introduced. We have developed a method for implementing MQMAS which accurately reflects quantitative information in the peak intensities [manuscript in preparation]. Examining the 23Na nuclei in a variety of compounds which contain multiple sites (sodium citrate dihydrate(3 sites), sodium dihydrogen phosphate (3 sites), sodium pyrphosphate decahydrate(2 sites)) our method, which combines rotation induced adiabatic coherence transfer with MQMAS, gives isotropic peaks which accurately reflect the populations as predicted by the x-ray studies of these compounds. Our findings are very encouraging that MQMAS can be developed for applications to complex biological systems. We will continue our work by examining the 17O MQMAS spectroscopy in a variey of model systems of increasing complexity and greater quadrupolar couplings to delineate how the isotropic chemical shifts and the quadrupolar couplings so obtained can be correlated to local atomic structure. Armed with this knowledge we will investigate the roles played by water molecules in the proton pumping mechanism of bacteriorhodopsin (bR) by inserting 17O-H2O into wild-type bR samples and recording the MQMAS spectra of bR in the dark- and light-adapted states as well as intermediate states. We note that in bR it will be crucial to be able to quantitate the MQMAS signals to facilitate assignments and to determine the number and location of water molecules in the bR pumping channel.